'Big G': Scientists Pin Down Elusive Gravitational Constant

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A fundamental constant that sets the size of the gravitational
force between all objects has finally been pinned down using the
quirky quantum behavior of tiny atoms.

The new results could help set the official value of the
gravitational constant, and may even help scientists find
evidence of extra space-time dimensions, said study co-author
Guglielmo Tino, an atomic physicist at the University of Florence
in Italy. [ Twisted
Physics: 7 Mind-Blowing Findings ]

Elusive value

According to legend, Sir
Isaac Newton first formulated his theory of gravity after
watching a falling apple. In Newton's equations, the force
of gravity grows with the mass of two objects in question,
and the force gets weaker the more distant the objects are from
each other. The English polymath knew the objects' masses had to
be multiplied by a constant, or "big G," in order to arrive at
the gravitational force between those two objects, but he wasn't
able to calculate its value. ("Big G" is different from "little
g," which is the local gravitational acceleration on Earth.)

In 1798, scientist Henry Cavendish calculated big G in order to
determine Earth's mass. To do so, Cavendish suspended dumbbells
on a wire, with enormous lead spheres placed at different
distances nearby, and then measured how much the dumbbells
rotated in response to the attractive pull of gravity from the
neighboring dumbbell. [ 6
Weird Facts About Gravity ]

Since then, almost every attempt to measure big G has used some
variation of Cavendish's method. Many of those experiments got
fairly precise values — which didn't agree with one another.
That's because it was too difficult to identify all potential
sources of error in the complicated systems used, said Holger
Müller, an atomic physicist at the University of California,
Berkeley, who was not involved in the new study.

"The gravitational force is just super tiny, so anything from air
currents to electric charges can give you a false result," Müller
told Live Science.

As a result, big G is known with much less precision than other
fundamental constants, such as the speed of light
or the mass of an electron, Tino told Live Science.

Keeping cool

The big systems didn't seem to be working, so the researchers
decided to go very small.

The team cooled rubidium
atoms to just above the temperature of absolute zero (minus
459.67 degrees Fahrenheit, or minus 273.15 degrees Celsius),
where atoms hardly move at all. The researchers then launched the
atoms upward inside a vacuum tube and let them fall, in what's
called an atomic fountain.

They also placed several hundred pounds of tungsten nearby.

To see how the tungsten distorted
the gravitational field, they turned to quantum mechanics,
the bizarre rules that govern subatomic particles. At small
scales, particles such as atoms can also behave like waves —
meaning they can take two different paths at the same time. So
the team split the paths the rubidium atoms took as they fell,
and then used a device called an atomic interferometer to measure
how the waveforms of those paths shifted. The shift in the peaks
and valleys of the paths when they recombined was a result of the
gravitational pull of the tungsten masses.

The new measurement of G — 6.67191(99) X 10 ^ -11 meters cubed /
kilograms seconds ^2 — isn't as precise as the best measures, but
because it uses single atoms, scientists can be more confident
the results aren't skewed by hidden errors that foiled the more
complicated setups of past experiments, Tino told Live Science.

The achievement is impressive, Müller said.

"I thought this experiment would be close to impossible, because
the influence of those masses [on gravitational pull] is just
very small," Müller told Live Science. "It's really a great
breakthrough."

New value

The new experiment raises the hope that future measurements can
finally settle on a more precise value for big G.

The findings also could help scientists discover if something
more bizarre is at play. Some theories suggest that
extra dimensions could warp the gravitational fields in our
own four-dimensional world. These distortions would likely be
very subtle and would only be noticeable at very small distances.
In fact, others have suggested that the different results other
labs have gotten were caused by this extradimensional intrusion,
Tino said.